Electrically heated pin-point gate

ABSTRACT

To provide a thermodynamic pinpoint gate nozzle which will permit a so-called cosmetic gate without stringing, and during which the plastic material to be processed is not subjected to high viscous or shear stress by the heating elements placed inside the nozzle channel, it is provided that the pinpoint gate nozzle on the side facing the mold cavity exhibits a low-mass inside nozzle (7) and that inside nozzle (7) can be heated by an ohmic resistance wire either inductively or by inside nozzle (7) formed as a Peltier element.

The invention relates to an electrically heated pinpoint gate nozzle.

A pinpoint gate for injection molds to produce plastic articles has theadvantage that no further processing is required, there is no waste andenergy is saved.

Since in principle the problem and the drawback is that the melt freezestoo soon in the narrow, cool nozzle channel, and thus only incompletedwell pressure can be exerted as a result of which the injection-moldedparts can exhibit bad dimensional stability, it is known in principle toequip the pinpoint gate system with heating of the gate channel. Forthis purpose, the gate channel is generally permanently heated, and asthe only variable, the temperature is adjusted as precisely as possible.

In the known heating system, for example according to DE-GM 85 35 572,the channel wall of the nozzle channel is heated by a heating coilsurrounding it. In other systems, a heated torpedo jutting with the tipinto the gate opening is used as mentioned, e.g., in the journal"PLASTverarbeiter, Vol. 33, 1982, pages 387 to 392." Because of the typeof heating, for reasons of stability, the heating with the conventionaltorpedo constructions cannot be brought into the immediate vicinity ofthe thermally critical gate area. For this reason, the torpedo tips areoften shaped over an extended length like pointed cones. Thecontinuously decreasing, heat-conducting cross section necessarily leadsto a larger temperature difference within the nozzle. The consequence isan increased risk of thermal damage to the melt flowing along thetorpedo.

Heating a torpedo tip mounted in the nozzle channel by a separatecontrol circuit has already been suggested and tried.

However, such a system is extremely wasteful and expensive and has notproved itself in practice.

Basically, a Japanese system in which the torpedo tip jutting into thegate opening is cyclically heated has also become known.

In the prior art initially cited, the heating of the hot well and of thepinpoint gate channel has the disadvantage that even when the injectionmolded part is demolded, the thermoplastic melt in the pinpoint gatestill becomes hot and, therefore, plastic. This disrupts the demoldingprocess, particularly with small parts.

In contrast to this, the prior art described in the second instanceusing heatable torpedoes can produce a so-called cosmetic gate since theoutside of the sprue has cooled sufficiently during demolding andtherefore breaks off cleanly. Nevertheless, this prior art has severalother decisive disadvantages compared to the hot well heating. Thus, thefact that the heated tip juts into the gate opening even furtherincreases the shear rate and thus the shear stress, which often reachesexcessive values even in the case of the open nozzle. This can lead tomechanical damage of the material. A thicker melt boundary layer thanthat on the hot channel wall forms on the cold side of the nozzlechamber, thus further narrowing the free opening cross section, whichshould anyhow be as small as possible for a cosmetic gate. This alsoleads to a greater undesirable increase in the shear stress. Inaddition, the exact fitting-in of the torpedo tip into the nozzleopening requires a large technical expenditure, which is also reflectedin the high cost of the system. Finally, if color or material arechanged, a large number of the reject moldings occur because thecomplicated melt path only permits the new material to slowly displacethe old.

It is the purpose of this invention to overcome the drawbacks accordingto the prior art and to produce a thermodynamic pinpoint gate nozzlewith which a cosmetic gate can be achieved, using the largest possibleflow opening to avoid viscous and shear stress.

In the pinpoint gate nozzle according to the invention, a low-massinside nozzle is provided in the nozzle point which is heated therewithout the torpedo heating placed inside the nozzle channel. Thus, theinside nozzle can be extremely rapidly heated and just as rapidly cooledand the nozzle channel is kept hot only during the injection and dwellphase, so that a desired sealing time can be set. During the coolingphase, the nozzle point is cooled by the cold tool (by means of thelug), thus allowing a smooth break during demolding. Using the cyclicheating, thus clearly reproducible conditions can be achieved. With thethermodynamic pinpoint gate nozzle, the following advantages can berealized:

A cosmetic gate with a smooth break without any pin drawing or stringingis possible at all times.

The pinpoint gate nozzle has the largest possible free flow opening. Byheating the channel walls during injection of thermoplastics, only athin boundary layer forms.

The nozzle channel is protected by a wear-resistant and anti-adhesivecoating of, e.g., titanium nitride or similar substances. As a resultmany plastic melts slide along the channel wall during injection. Thesliding entails slight shear forces and shear stress, thus protectingthe material.

No parts whatsoever to be inserted are planned that could increase theshear stress or jut into the nozzle opening.

The nozzle has no moving parts which could jam, are subjected to wear orneed to be set with highest precision.

The hot channel walls only briefly come into contact with movingthermoplastic melt. As soon as the melt-flow stops, the heating isturned off. Therefore, this does away with the need for a costlytemperature monitoring system and the nozzle is particularly suitablefor thermally sensitive plastics.

There are no undercuts or dead corners in the melt path.

A change of material or color can easily be accomplished without wasteby the removal of the cold slug from the hot well.

By dimensioning the nozzle opening, the throughflow for multicavitymolds can be regulated.

The nozzle heating system uses relatively little current.

The construction of the nozzle is simple and it can be quickly installedor removed.

The nozzle is relatively small and compact. Its installation causes onlyminor weakening of the mold plate.

In this connection, the heating system can be operated either by aresistance wire inductively or by a Peltier element.

Further advantages, details and features of the invention follow belowfrom embodiments of the drawings represented. Here are shown in detail:

FIG. 1: a diagrammatic longitudinal section through a pinpoint gatenozzle according to the invention, heated by a coil or a resistancewire;

FIG. 2: a modification of FIG. 1 in which the heating of the insidenozzle takes place by two contact pins;

FIG. 3: another diagrammatic cross section representation of a pinpointgate nozzle according to the invention, in which the inside nozzle isheated by a Peltier element.

FIG. 1 shows a pinpoint gate nozzle with a hot well 3 in nozzle body 1.An inside nozzle 7, narrowing toward gate opening 5, is provided at theside of nozzle body 1 facing a bottom mold cavity in a mold, not shownin detail in FIG. 1. The inside nozzle consists of a material with goodthermally and electrically conductive properties of a high resistance towear and a low mass of, for example, less than 5, 4, 3 or 2 grams, andpreferably less than 1 gram. The outside of inside nozzle 7 issurrounded or coated by a material which is both thermally insulatingand a poor conductor of electricity. A ceramic, for example zirconiumoxide, is suitable for this purpose. Naturally, other materials or otherceramic materials are suitable as thermal insulation 9.

Further, one of two connecting wires 11 is shown in FIG. 1. The furthercourse of connecting wire 11 for the power supply for heating is shownonly diagrammatically as a broken line in FIG. 1 and leads to a spiralor coil 13 embedded in the thermal insulation material, preferably inceramic 9. Spiral or coil 13 can act as a thermal coil, that is, as aresistance wire, to provide heat. But just as possible is inductiveheating by an alternating current field of suitable frequency whichheats inside nozzle 7.

When the inside nozzle 7 is rapidly heated, the heating process and thecourse of the temperature are influenced by the thickness of lug 15,i.e. of metal lug (15), at the front end of the nozzle, which thermallyconnects the nozzle point with the mold, not shown in FIG. 1, or by thethermal insulation layer, i.e. the ceramic layer, which insulates usualnozzle body 1 of the mold. In this way the heat loss during the coolingperiod can be preset and established. Three magnitudes can thus bevaried in the case of thermodynamic pinpoint gate(s). They are: thermaloutput (temperature), length of the heating period and heat loss.

The following refers to FIG. 2, in which inside nozzle 7 is heated bytwo contact pins 17 to produce an ohmic resistance. Deviating from theembodiment of FIG. 1, heating here is achieved by a defined ohmicresistance rather than by a thermal coil or inductive heating. Insidenozzle 7 itself is not suitable for this because its cross section istoo large. The two contact pins 17 mentioned are therefore used asresistors, and are mounted near the narrowest inside cross section ofthe nozzle with their diameter (approx. 1 mm) being exactly maintained,and connected to the electrical heating circuit by the two connectingwires 11. Since the other cross sections of the heating circuit are muchlarger, less than 15 volts will produce heat in these contact pins whichwill immediately be conducted to nozzle body 1 because of the goodthermal conductivity.

The heating circuit is automatically switched on or off by the injectionmolding machine during opening or closing of the mold. The length of theheating period can then be adjusted in an infinitely variable manner,e.g. by a time switch. Further, the strength of the current, whichdetermines the temperature, is regulated. When the system switches on,the strength of the current is increased by suitable means, so that thetemperature rapidly increases at the beginning, thus creating a largerelectrical resistance. But other means can, of course, also produce aninitially stronger, then gradually decreasing thermal output. Afterheating, the heat loss during the cooling phase is so large that themelt temperature in the nozzle channel reaches the brittleness rangeduring demolding.

This means that inside nozzle 7 is not ideally thermally insulated andthat the heat loss can be influenced by the type of thermal insulation.This heat loss is overcompensated for by a correspondingly large thermaloutput during the heating phase. Since the thermal value of insidenozzle 7 is regulated in this manner, the required thermal output (amaximum of 200 watts) can easily be possible, which can be achieved bythe above-noted defined ohmic resistance with appropriate use of contactpins.

Further in FIG. 2 in addition a sealing ring can be provided on ashoulder in the area toward the mold. The nozzle body itself can beprovided on the outside with continuous recess rings to achieve an airgap 21. Also in the embodiment explained here, above mentioned contactpins 17 are embedded in the thermal insulation body, preferably in theceramic body, and are in contact with the inside nozzle with formationof a common heating circuit.

Finally reference is made to FIG. 3, in which heating by means of thePeltier effect is shown. For this purpose, the inside nozzle must bethermally separated as well as possible from the rest of the cooledmold, which is why the inside nozzle can be surrounded by a front andrear sealing ring in the direction of flow. It is further sheathed witha ceramic insulation as thermal insulation 9 and preferably made ofzirconium oxide, with low thermal conductivity. The mounting can also bebetter thermally separated than with ohmic heating. The nozzle itself isformed as Peltier element and consists of three conical inside nozzlebodies sitting inside one another with a central conductor A to whichinside and outside a conical conductor 8 each has been soldered. Thermalinsulation 9 is surrounded by steel ring 19.

Depending on the polarity of the direct current, the innermost insidenozzle body will either be heated or cooled by the passage of thecurrent. Thus, since in the case of inductive or ohmic heating, adirectly controllable cooling effect is possible, the size anddimensioning of lug 15 mentioned in the other systems and thermalinsulation 9 have only little importance for regulating the heat lossduring the cooling phase. As shown in FIG. 3, lug 15 can even be omittedin this embodiment.

The invention concerns a pinpoint gate nozzle exhibiting a heretoforeunknown additional inside nozzle 7 which is separately heated. Hereheating takes place especially by inductive heating or by heating bymeans of a Peltier element in the inside nozzle, that is, in the wall ofthe inside nozzle itself. When ohmic resistance heating is used, theheat is supplied by resistance coils or wires surrounding the outsidecircumference of the inside nozzle, for example in the form of the abovementioned contact pins. With suitable construction corresponding ohmicresistance elements could optionally also be provided in the wall of theinside nozzle.

When the injection nozzle is used in multicavity molds, the hot well isomitted. In multicavity molds with several, also differing mold volumes,the filling rate can be regulated by variation of the nozzle dimensionsor the nozzle opening. Thus, e.g. simultaneous filling of several moldcavities of different size can be achieved by providing the smaller moldcavity with a pinpoint gate nozzle of smaller dimensions, and providingthe larger mold cavity with a correspondingly larger nozzle. The heatedrunner system can then have an equal channel cross section throughout.When thermoplastics are to be processed, the nozzle is heated during theinjection phase. When elastomers or duromers are to be processed, thenozzle is not heated during the injection phase, but rather, it isheated during the following crosslinking reaction.

I claim:
 1. An electrically heatable pinpoint gate nozzle for aninjection molding machine said gate nozzle comprising:means defining anaxially extending central flow channel having a gate opening forallowing flow of plastic material into a mold cavity of a moldingmachine; a conical inside nozzle in said flow channel formed of athermally conductive wear-resistant material and located adjacent saidgate opening on an upstream side thereof; said inside nozzle surroundingsaid gate opening and having a central flow path in communication withsaid flow channel and said gate opening; means carried by said gatenozzle for heating the inside nozzle; and a lug carried by the nozzleand formed of thermally conductive material, said lug being disposedperpendicular to a longitudinal axis of said flow channel and in thermalcontact with said inside nozzle, whereby heat transfer during a coolingperiod is dependent upon the thickness of the lug.
 2. A nozzle accordingto claim 1 wherein the mass of the inside nozzle is on the order of 5 gor less.
 3. A nozzle according to claim 1 wherein said flow channeldefining means includes a nozzle body, and means for thermallyinsulating said inside nozzle from said nozzle body.
 4. A nozzleaccording to claim 3 wherein said thermal insulating means comprises anon-electrically conductive material.
 5. A nozzle according to claim 4wherein said non-electrically conductive material is a ceramic.
 6. Anozzle according to claim 1 wherein said heating means includes aresistance wire disposed about said inside
 7. A nozzle according toclaim 6 wherein said heating means includes two or more contact pinsattached to said inside nozzle outside said nozzle flow channel forheating said inside nozzle by ohmic resistance heating.
 8. A nozzleaccording to claim 1 wherein said heating means includes a coil windingsurrounding said inside nozzle for inductively heating said insidenozzle.
 9. A nozzle according to claim 3 wherein said heating means isembedded in said thermally insulating means surrounding said insidenozzle.
 10. A nozzle according to claim 1 wherein said heating means islocated to provide heat to the plastic material flowing through saidgate opening.
 11. An electrically heatable pinpoint gate nozzle forinjecting plastic material into a cavity of a molding machine, said gatenozzle comprising means defining a nozzle body having a central flowchannel for the plastic material, a low mass inside nozzle locatedinside said central flow channel and facing the mold cavity, said insidenozzle including a Peltier element for heating said inside nozzle andmeans for supplying electrical energy to said Peltier element.
 12. Anozzle according to claim 11 including means for supplying a directelectrical current to said Peltier element whereby said inside nozzlecan be either heated or cooled, depending on the polarity of the directcurrent, and means for thermally insulating said inside nozzle relativeto the nozzle body.
 13. A nozzle according to claim 11 wherein the massof the inside nozzle is on the order of 5 g or less.